How to prepare for success when painting galvanised steel

How to prepare for success when painting galvanised steel

Tuesday 2 March 2021


If you are ever faced with the task of painting galvanised steel, some preparation may be needed to help ensure an effective job. In this article we look at why this is so, and the type of preparation required. But first, a little history…

In 1836, Stanislas Sorel in France took out the first of numerous patents for a ‘galvanic’ process of coating steel with molten zinc. The first UK use of galvanised corrugated iron is believed to have been for the Navy in 1844. Today galvanised metal is all around us, used in construction, transport, agriculture and everywhere that good corrosion protection is essential.

Coating steel with molten zinc prevents premature rust and corrosion because zinc corrodes very slowly, and therefore protects the steel’s surface. Due to the zinc being of lower nobility than steel - see our blog ‘Why do we use Zinc in our primers?’ - the zinc becomes sacrificial to the steel. (Zinc becomes the anode and steel becomes the cathode, creating cathodic protection.)

Does galvanised steel rust?

The answer is yes and no. Over many years, the deadly orange or brown rot will creep in, but very slowly and not as clearly visible as when non-galvanised steel rusts. Of course, there are a lot of factors that require answering when trying to determine how long the galvanising will last. These include the environment, whether there is contact with other metals, and how thick the zinc was applied. Clearly galvanising a metal surface with 125μm (microns) of zinc is better than 25μm, but also 5x the cost.

The graph below shows the relationship between zinc thickness and the corrosion rate:

 

Graph showing life expectancy of galvanised steel

Should you paint galvanised steel?

Painting galvanised steel is often done for aesthetic reasons, but painting also helps to maintain its performance. In fact, maintaining galvanised steel is just as important as maintaining other steel. Why go to the extra cost of galvanising and then let it fail due to a lack of maintenance?

Galvanised coatings can be painted with a variety of paint types. Rustbuster has a range of options, including our EM121 Epoxy Rust Proofing Paint, Custom 421 Epoxy Primer, Cold Galvanising Paint and Zinc Rich Epoxy Anode.

Can you just paint straight over galvanised metal?

This is another ‘yes and no’ answer…

Yes, if the galvanised metal is several years old, so that natural weathering has removed surface greases left from the manufacturing process.

No, if the galvanising is fresh and hasn’t been subject to extensive weathering. In this case, the surface grease will need to be removed first.

Preparation before painting galvanised steel

Rustbuster offers a two-step system for degreasing and neutralising galvanised steel prior to painting:

Step 1 - Degrease. Use either Rustbuster’s Safer De-Greaser (available in direct-to-surface and concentrated formulas) or SP10 Tank-Kleen caustic detergent (which should first be diluted with water in a 1:4 ratio).

Apply your chosen chemical to the surface by brush or spray. Work it in to the surface with a brush to create a foam. Leave for 5-60minutes to allow the product to work. Then wash away with Rustbuster’s Chlor-X Salt Remover or fresh water before allowing to dry.

Step 2 – Apply T-Wash. Rustbuster’s T-Wash galvanising treatment (mordant solution) turns grease-free galvanised steel black or grey to indicate that it is ready to paint. It also neutralises the hydroxides and carbonates on the galvanised surface.

Apply T-Wash evenly by brush and give it time to react. The reaction will create a black or dark grey appearance. You should wash the surface again with Chlor-X or fresh water before allowing to dry prior to painting.

If you don’t see a reaction indicated by the colour change, repeat degreasing as described in step 1, followed by another application of T-Wash.

Following this process should ensure you have a surface fully prepared to achieve excellent adhesion between the steel and your chosen paint.

Should you require further assistance with preparing galvanised steel for painting, please don’t hesitate to contact us. And to be the first to hear about further blogs, videos and other useful content, please sign up to our email newsletter using the form below.

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